There is proposed a technique to control electric power supply-demand balance as a whole power system. In the technique, a power supplier generating or supplying power requires power consumers to reduce power consumption when the power supplier operates various power systems.
However, a time-lag takes place between the requirements of power reduction by the power supplier and actual power reductions by the power consumers. The time-lag causes a problem that a robust control of power supply-and-demand balance cannot be performed particularly when an amount of power supply and demand varies with time.